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The influence of gas and liquid physical properties on entrainment inside a sieve tray column

Uys, Ehbenezer Chris (2012-12)

Thesis (PhD)--Stellenbosch University, 2012.

Thesis

ENGLISH ABSTRACT: Distillation column design and operation require understanding of both the hydrodynamic
and thermodynamic behaviour and limitations. One of the hydrodynamic aspects that
negatively influence separation efficiency in the distillation column is entrainment of the
liquid with the rising vapour or gas. Inaccurate entrainment predictions will lead to poor
separation efficiencies in the column and consequently over design of the column diameter
and/or height has to be incorporated. This has a significant impact on the capital cost due to
the size and scale of industrial columns. Therefore, small improvements in entrainment
prediction will lead to large savings in capital investment.
Previous research published in the open literature focused primarily on the influence of gas
and liquid flow rates and, tray geometry on entrainment for the air/water system.
Consequently the non-air/water database is small and consists of data obtained from
various tray and column geometries. As a result the accuracy of current entrainment
prediction models is questionable for systems other than air/water. Therefore, the first
objective of this work was to investigate whether current prediction models perform well
for systems other than air/water. To prove this air/water, air/ethylene glycol and air/silicon
oil data were measured and compared with current prediction correlations. It was found
that current prediction models perform poorly for the air/ethylene glycol and air/silicone oil
systems. At the same time a new observation was made with regard to froth development
and behaviour inside the column. The observation shows that liquid flow rate has a nonmonotonic
influence on entrainment, caused by the short (475mm) tray flow path.
The second objective was to examine the influence of gas physical properties on
entrainment. New entrainment data were measured by individually contacting air, CO2 and
SF6 with water and ethylene glycol, while n-butanol was contacted with CO2 and SF6. The
data was compared with current prediction models which performed poorly for SF6 results.
This shows the inability of these models to predict entrainment for gas systems with high
densities. Modified Reynolds and Froude numbers were developed to show the influence of
gas physical properties on entrainment. Low modified Reynolds numbers and large modified
Froude numbers resulted in high entrainment.
The third objective was to determine the influence of liquid physical properties on
entrainment. New entrainment data were measured using CO2 with Isopar G, n-butanol,
water, silicone oil and ethylene glycol. Current prediction models compared poorly to the
data and did not include the influence of liquid viscosity on entrainment. It was found that
viscosity had an intricate non-monotonic influence on entrainment.
The fourth and final objective was to correlate the influence of gas and liquid properties on
entrainment as determined by the previous two objectives. To make the dataset more
complete, entrainment was measured for four tray spacings using CO2/Isopar, CO2/nbutanol,
air/ethylene glycol, CO2/ethylene glycol, air/silicone oil and CO2/silicone oil (over
1700 data points). Two new correlations are presented to predict the fraction of liquid
entraining with the rising gas (L’/G with R2 = 85%) and the fraction of liquid entering the tray
that entrains (L’/L with R2 = 92%). The performance of the L’/G correlation (R2 = 85%) is
vastly superior to two other prominent correlations (R2 = 61% and 23%). This correlation can
be implemented to predict entrainment successfully for different tray geometries by
combining the predicted influence of tray geometry, by Kister and Haas (1988), with results
from the newly developed correlation. All four objectives are presented as manuscripts for
journal publication and serve as alone standing documents.